The present invention relates generally to ultrasound devices and more particularly to methods of operating large annular transducer arrays such as those used in medical ultrasound diagnostic equipment.
It is well known to use ultrasound equipment to examine the interior of an object such as a human patient. The ultrasound equipment radiates high frequency ultrasonic pulses toward a patient and these pulses are reflected by boundaries between body organs which have different tissue density and acoustic elasticity (different acoustic impedance). The reflected pulses are detected to obtain diagnostic information such as a 1-dimensional scan, 2-dimensional tomographic image, or 3-dimensional volume image of the organs. The ultrasound diagnostic method is relatively non-invasive so that the patient suffers little discomfort. Ultrasound does not expose the patient to substantial radiation risks as compared to conventional x-ray diagnostic equipment while efficiently imaging soft tissue.
Though the ultrasound equipment is principally used as medical diagnostic equipment, any material that has a varying acoustic impedance can be examined by ultrasound equipment.
Piezoelectric transducers or other suitable transducers transmit beams of ultrasonic energy toward an object of study and detect the reflections or echoes produced by the object. To complete the acoustic examination, the ultrasonic beam is mechanically and/or electronically scanned over the area of interest. Many electronically scanned ultrasound systems utilize linear or annular arrays of transducers. The transducer elements are excited to generate acoustic beams at many angles relative to the normal line of the array at its midpoint. Echoes returning from the object arrive at the transducer elements at different times necessitating the steering of the beam by delaying the received echo signals by different amounts so that all the signals from a given target point of the object are simultaneously summed. In addition to beam steering delays, electronic focusing is provided by delays to compensate for propagation path time delay differences from the transducer elements to the focus point. Thus, the transducer arrays focus or steer the acoustic beam, by the use of time delays or phase shift methods in the pulse transmission of the acoustic beams. The amount of beam steering and focal point position are governed by the amount of phase delay between the pulses transmitted by the transducer elements of the array.
In an effort to sharpen the focus of the transmitted beam and thereby increase the spatial resolution of information contained in reflected beams, large arrays are being used to generate and receive the ultrasound pulses. In an annular array transducer of large diameter (e.g., a disk diameter of 30 mm) and working at high frequencies (e.g., 4.5 MHz in a water medium) the width of the acoustic beam becomes very narrow. An example of the lateral resolution of an acoustic beam 10 generated by a transducer 12 is shown in FIG. 1a. For the above example, the frequency width at the half-maximum (FWHM) level is 0.64.degree., as represented by the beam profile 14. Referring to FIG. 1b, the transmitted beam 10 (also known as a drive beam or interrogation beam) propagates through a medium until it interacts with acoustical features such as targets T.sub.1 and T.sub.2 located along the beam axis 16. The reflected or echo pulses 18 are received by the transducer array at distinct time intervals which correspond to the respective depths of the targets in the object.
In order to provide a scan image of the target, the transducer 12 can be mechanically scanned by rotating it about an axis. FIG. 2 depicts a transducer array that is rotated about an axis 20 (perpendicular to the plane of the figure) to provide a mechanical scanning operation. The mechanical movement results in a loss of sensitivity due to angular position error. Angular position error is caused by the mechanical scanning motion of the transducer array during a time t.sub.echo or interrogation time. Time t.sub.echo is defined as the time period required for the echo of the transmitted pulse to be received by the array. Thus, a drive pulse 10 is transmitted while the transducer array is in the position indicated by the dash-line outline. The array continues its scanning motion and at a time t.sub.echo, the echo pulse is received while the array is in the position indicated by the solid line outline. The loss of sensitivity is due to the fact that the reflected pulse 18 is now off-axis with respect to the transducer. Since the line normal to the array disk represents the direction of maximum sensitivity of the transducer array, any angular shift of the target away from the normal line represents a degeneration of the sensitivity of the system. In the receive mode, the echo pulse 18 interacts with the off-axis portion of the transducer array's beam profile, also depicted as line 14 for simplicity, and the detection of the received pulse is more difficult. It can be appreciated that as the size of the array is increased and the width of the beam becomes narrower, the loss of sensitivity will be greater. In the illustrated example, the received signal is about 10 dB weaker than a signal received in a non-scanned environment.
Another problem addressed by the present invention lies in the manufacturing process of the annular transducer array. The individual segments of the transducer array require a large contact area with the electrical leads and careful alignment with each other so that the segments and the contact areas do not contact each other (thereby creating destructive cross-talk) but are as closely adjacent to each other as manufacturing tolerances permit (thereby increasing relative resolution). Since the array ultimately includes at least four layers of material, namely a mounting cup, contact areas, piezoelectric crystal and an acoustic matching material, respectively, alignment of these layers can become arduous.